Cable is widely used for industrial equipment/devices to transfer working load. Working load can be power, analogue data or digital data. When a cable gets fatigued, it can no longer transfer well and will possibly make the equipment/device breakdown.
Cable fatigue is caused by cable-bending and/or cable-twisting occurred in a motion system, such as in an industrial robot. An industrial robot (manipulator) has several joints. When it works, its joints rotate quickly, thus the cables in it or attached to it bend or twist quickly. Therefore, an industrial robot is easier to have its cables fatigued, thus the risk of breakdown is high.
A conventional solution to cable fatigue is prediction, i.e. to predict the fatigue-life of a cable and replace it just before it cannot work. The prediction cannot be accurate enough because of the reasons:
According to the S-N curve (S means stress, N means the number of bending/twisting before a component breaks. S-N curve is a curve illustrating the relation between the stress applied on a component and the fatigue-life of the component), a little change of stress can cause a big change of fatigue-life. For example, 3% change of stress causes 30% change of fatigue-life. However, it costs too much to simulate the stress accurately enough, thus a large error of fatigue-life is unavoidable for practical operation.
The anti-fatigue performance (the relation between stress and fatigue-life) of any two cables is different, even if the two cables are of the same batch, by the same factory. The difference will be larger if the two cables are made by different factories. It is impractical for a robot producer to test the cables' anti-fatigue performance frequently. Therefore, a robot producer does not know exactly the anti-fatigue performance of the cables because the cables are of different batches or made by different cable suppliers. The result is that even if we simulate the stress very accurately, we still cannot predict the fatigue-life accurately.
The other conventional solution is to monitor cable fatigue and replace the cable when it cannot work well. It is more practical than prediction and there have been some attempts. The patent Arc Welding Method and Apparatus with Power Cable Fatigue Monitor (Patent No. US005911893A) gives a method to monitor cable fatigue, i.e. monitoring the voltage drop over the power cable. The power cable should retire when the voltage drop exceeds a threshold. The drawbacks of this patent include:
Considering the small resistance of a power cable, a very large current is necessary to make the voltage drop measurable. However, it is impossible to transfer so large current over a normal cable. Therefore, this patent is only applicable for arc welding power cable. It is not suitable to most applications such as data cable or motor power cable.
This patent introduces new cables (component 66 and 67 in FIG. 7 of this patent) to measure the voltage drop, which increase the risk of fatigue.
The patent Welder Cable Monitor (Patent No. US005637241A) is similar to the previous patent.
There are several existing patents to monitor with mechanical performance. Of course the component can be a cable. Here are some of them:
Remote and Powerless Miniature Fatigue Monitor and Method (Publication No. U.S. Pat. No. 5,531,123A);
Fatigue Monitor for Small-diameter Piping Joint (Publication No. JP61110029A);
Monitor Material, Method for Measuring Fatigue Damage Degree Using the Same, and Machine Part with Monitor Material (Publication No. JP2001201432A);
Fatigue Damage Monitor Gauge and Fatigue Damage Monitor Device (Publication No. JP10111267A);
Monitor for Detecting Fatigue Damage Monitor (Publication No. JP5113390A)
Fatigue Monitor of Rotating Shaft (Publication No. JP54116986A); and
Strain Gauge Having Double Function as Fatigue Aging-monitor (Publication No. JP7218214A).
These patents still have some drawbacks because:
What they monitor is the mechanical performance but not the electrical performance. Usually during the process of fatigue, a cable loses its electrical performance sooner than losing its mechanical performance, i.e. it is not suitable to transfer power/data but the degradation of its mechanical performance is very little. Hence, a cable may be eligible according to these patents but in fact it should retire.
Some of these patents attach a piece of monitor material to the monitored component. The monitor material is a reference to estimate the fatigue damage on the monitored component. However, a robot doesn't have too much room for the monitor material. It is not suitable to detach the monitor material for testing because the robot is always busy. Besides, the difference between the monitor material and the monitored component introduces a new error. What's more, if an industrial robot manufacture wants to apply these patents, it must turn to a cable supplier for cable with monitor material. Hence, the robot producing company might lose its vantage point in business.
According to the other conventional technologies, engineers test the fatigue-life of a cable by comparing the initial resistance of a cable with the value after bending/twisting. A large enough degeneration means that the cable is fatigued. For example, the initial resistance of a cable was 0.493 ohm. The resistance became 0.702 ohm after bending/twisting the cable for 3,818,090 times. Hence, the engineers know that the fatigue-life of this batch of cable is about 3.8 million cycles. However, there is no working load over the sample cable and the engineers can measure the resistance with a multimeter. For a cable installed in a robot, the working load over it will disturb the multimeter so the resistance is not measurable.